A. As a ball screw enables reduction of friction coefficient in rotating the screw shaft relative to the nut as compared with a sliding contact type screw, it has become commercially practical in various fields such as positioning mechanism of machine tools, vehicle steering, guide devices, motion screws and the like. As illustrated in FIG. 30, a return pipe type ball screw has a plurality of balls 3 arranged in a loaded ball rolling path between a spiral ball rolling groove 1a on the outer peripheral surface of a screw shaft 1 and a spiral loaded ball rolling groove 2a on the inner peripheral surface of a nut 2, and the balls 3 are circulated in a ball return path of a U-shaped return pipe 4 (circulation member) formed in the nut 2 (for example, see patent document 1). In the side surface of the nut 2, a return pipe insertion hole 2b is formed which extends up to the loaded ball rolling groove 2a. When a leg part 4a of the return pipe 4 is inserted in the return pipe insertion hole 2b, one end of the spiral loaded ball rolling groove 2a of the nut 2 and the other end thereof are connected via the ball return path of the return pipe 4. The plural balls 3 arranged between the nut 2 and the screw shaft 1 circulate in a ball circulation path formed of the spiral loaded ball rolling groove 2a and the ball return path in the return pipe 4.
Each ball 3 moves from the loaded ball rolling groove 2a of the nut 2 to the ball return path of the return pipe 4, and vice versa. At this time, if there is any elevation change (step height) at a joint of the loaded ball rolling groove 2a of the nut 2 and the ball return path of the return pipe 4, the ball 3 is prevented from moving smoothly. Accordingly, as shown in FIG. 31, in order to eliminate the elevation change at the joint 6 of the ball return path 5 and the loaded ball rolling groove 2a, a connecting part 7 of the loaded ball rolling groove 2a to the ball return path 5 is subjected to chamfering. Strictly speaking, in case of a ball screw using balls as rolling elements, the cross section of the loaded ball rolling groove 2a is shaped like Gothic arch composed of two arcs so that each of the balls is in contact with the loaded ball rolling groove 2a at two points. On the other hand, the cross section of the ball return path 5 is shaped like a single arc. The connecting part 7 is also chamfered so that the cross section of the loaded ball rolling groove 2a is changed gradually to conform to the cross section of the ball return path 5 at the joint 6 thereof.    [Patent Document 1] Japanese Patent Laid-open Publication No. 2000-18359
B. FIG. 32 illustrates a deflector type ball screw. A nut 71 has a side surface, in which a deflector hole 72 is formed. A deflector 73 is fit in the deflector hole 72. The deflector 73 has an S-shaped return groove 73a formed therein connecting both ends of a spiral loaded ball rolling groove 71a of less than one turn formed in the inner peripheral surface of the nut 71. This return groove 73a acts to move each ball 75 away from a ball rolling groove 76a of a screw shaft 76, make the ball 75 go over a thread 76b of the screw shaft 76 and return the ball 75 one turn back to the ball rolling groove 76a. The deflector 73 is a resin molding, and it is fit in the deflector hole 72 of the nut 71 and secured to the nut 71 by an adhesive agent.
Meanwhile, the applicant has proposed a wing type deflector 82 which can be inserted into a nut 81 without any adhesive agent, as shown in FIG. 33 (see the patent document 2). The wing type deflector 82 has a deflector main body 82a having a return groove 82c formed therein and a wing part 82b which juts from a side surface of the deflector main body 82a and fits into the loaded ball rolling groove 81a of the nut 81. Between the return groove 82c of the deflector 82 and the screw shaft 84, there are balls 85 arranged so that the deflector 82 is prevented from falling into the screw shaft 84 side.    [Patent Document 2] Japanese Patent Laid-open Publication No. 2002-106672